giostreamsink: Add a property to close stream on stop()

Back in the mists of time[1], we switched giostream* elements to not close the stream on stop() so that applications that needed a handle to the stream after the element stopped had it.

Unfortunately, we also have cases[2] where waiting for the element to be finalized is too late for the stream to be closed.

In order to not change the behaviour of the element, we add a property to allow users to select the desired behaviour.